Ganong's Review of Medical Physiology, 23rd Edition

316 SECTION IV Endocrine & Reproductive Physiology
and glucagon excess makes diabetes worse. Excess pancreatic
production of somatostatin causes hyperglycemia and other
manifestations of diabetes.
ISLET CELL STRUCTURE
The islets of Langerhans (Figure 21–1) are ovoid, 76- × 175μm
collections of cells. The islets are scattered throughout the
pancreas, although they are more plentiful in the tail than in
the body and head. β-islets make up about 2% of the volume
of the gland, whereas the exocrine portion of the pancreas (see
Chapter 26) makes up 80%, and ducts and blood vessels make
up the remainder. Humans have 1 to 2 million islets. Each has
a copious blood supply; blood from the islets, like that from
the gastrointestinal tract (but unlike that from any other endocrine
organs) drains into the hepatic portal vein.
The cells in the islets can be divided into types on the basis
of their staining properties and morphology. Humans have at
least four distinct cell types: A, B, D, and F cells. A, B, and D
cells are also called α, β, and δ cells. However, this leads to
confusion in view of the use of Greek letters to refer to other
structures in the body, particularly adrenergic receptors (see
Chapter 7). The A cells secrete glucagon, the B cells secrete
insulin, the D cells secrete somatostatin, and the F cells
secrete pancreatic polypeptide. The B cells, which are the
most common and account for 60–75% of the cells in the
islets, are generally located in the center of each islet. They
tend to be surrounded by the A cells, which make up 20% of
the total, and the less common D and F cells. The islets in the
tail, the body, and the anterior and superior part of the head
of the human pancreas have many A cells and few if any F
cells in the outer rim, whereas in rats and probably in
humans, the islets in the posterior part of the head of the pancreas
have a relatively large number of F cells and few A cells.
FIGURE 21–1 Islet of Langerhans in the rat pancreas. Darkly
stained cells are B cells. Surrounding pancreatic acinar tissue is lightcolored
(× 400). (Courtesy of LL Bennett.)
A variety of other hormones also have important roles in
the regulation of carbohydrate metabolism.
The A-cell-rich (glucagon-rich) islets arise embryologically
from the dorsal pancreatic bud, and the F-cell-rich (pancreatic
polypeptide-rich) islets arise from the ventral pancreatic
bud. These buds arise separately from the duodenum.
The B cell granules are packets of insulin in the cell cytoplasm.
The shape of the packets varies from species to species;
in humans, some are round whereas others are rectangular
(Figure 21–2). In the B cells, the insulin molecule forms polymers
and also complexes with zinc. The differences in the
shape of the packets are probably due to differences in the size
of polymers or zinc aggregates of insulin. The A granules,
which contain glucagon, are relatively uniform from species
to species (Figure 21–3). The D cells also contain large numbers
of relatively homogeneous granules.
STRUCTURE, BIOSYNTHESIS, &
SECRETION OF INSULIN
STRUCTURE & SPECIES SPECIFICITY
Insulin is a polypeptide containing two chains of amino acids
linked by disulfide bridges (Table 21–1). Minor differences
occur in the amino acid composition of the molecule
from species to species. The differences are generally not
sufficient to affect the biologic activity of a particular insulin
FIGURE 21–2 Electron micrograph of two adjoining B cells
in a human pancreatic islet. The B granules are the crystals in the
membrane-lined vesicles. They vary in shape from rhombic to round
(× 26,000). (Courtesy of A Like. Reproduced, with permission, from Fawcett DW:
Bloom and Fawcett, A Textbook of Histology, 11th ed. Saunders, 1986.)